We describe a cryogenic two-terminal high-resistance bridge and its application in precision resistance scaling from the quantized Hall resistance (QHR) at RH = RK/2 = 12 906.4035 Ω to decade resistance standards with values between 1 MΩ and 1 GΩ. The design minimizes lead resistance errors with multiterminal connections to the QHR device. A single variable voltage source and resistive ratio windings are utilized to achieve excellent dynamic stability, which is not readily obtained in low-current measurements with conventional cryogenic current comparators (CCCs). Prototypes of this bridge have been verified by a successful international comparison of high-resistance scaling using two-terminal CCCs in the national metrology institutes of Argentina, Mexico, and the United States.
In the last few years we have been working in order to obtain a Resistance Standard using the Quantum Hall Effect (QHE). Several AlGaAs/GaAs structures and a variety of ohmic contacts procedures have been evaluated to achieve devices with metrological quality. Recently we have obtained devices with the following characteristics: The resistance plateau corresponding to the quantum number i = 2 is obtained for a magnetic flux density in the order of 7 T at 1.3 K, the residual value of the longitudinal resistance is 107 µΩ for a current of 50 µA. These characteristics are comparable with those obtained in QHE devices used at Centro Nacional de Metrologia (CENAM). Such results show that the developed devices have the quality required for reference standard dc resistance measurements.
This paper describes the development of bifilar, quadrifilar, and octofilar Calculable Resistors (CRs). The research involves Evanohm-S and Isaohm wire heat treatment processes to achieve temperature coefficients less than 0.5 µΩ/Ω/ • C in the CR's wire resistance element, tests of different terminal-wire joining techniques, and construction aspects achieving a stability of less than 0.05 µΩ/Ω/day. This kind of construction methodology has not been presented in detail in previous CR papers, and it is essential to accomplish the correct parameters of a CR. Without it, the development of a CR can take several months or even years. A comparison between CRs developed in this research and a CR from the Federal Institute of Metrology (METAS) in Switzerland was carried out. Measurement results between the 10 kΩ octofilar CR and the METAS 1 kΩ coaxial CR show an agreement better than 0.35 µΩ/Ω through the audio-frequency range. Therefore, the octofilar CR can be used as an AC resistance reference with traceability to the quantum Hall resistance in DC.
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